This invention relates to tunable filters and Fabry-Perot etalons.
Wavelength-division multiplexing (WDM) systems are an efficient means for increasing the transmission capacity of telecommunications networks. WDM refers to simultaneously sending multiple signals through a single fiber, wherein each signal has a different wavelength. Separating different signals in a WDM system requires filters having transmission and/or reflection bands that are at least as narrow as the wavelength different between adjacent signals.
A Fabry-Perot etalon is an example of an optical filter. A Fabry-Perot etalon is an optical device that can transmit light at a series of discrete wavelength bands. Light entering the Fabry-Perot etalon enters an optical cavity that is bounded by a pair of reflective surfaces. The reflective surfaces are separated by a precisely controlled distance that determines a set of transmission wavelengths for the filter. The transmission properties of a Fabry-Perot etalon are dependent on parameters such as the reflectivity of the reflective surfaces and the separation of the reflective surfaces. For example, the higher the surface reflectivity, the narrower the transmission bands. Also, the smaller the separation, the further apart the transmission bands are in wavelength. That is, the smaller the separation, the larger the free spectral range (FSR) of the filter.
A tunable Fabry-Perot etalon adds an adjustable component to the separation by which the peak wavelengths of the transmission bands can be changed. Tuning can be achieved in a Fabry-Perot etalon by making one of the two reflectors a movable or deformable membrane and applying a voltage between the membrane and the second fixed reflector, thereby changing the cavity separation distance through electrostatic attraction. In such a device, the amount of deflection and, therefore, cavity length control, is dependent upon the distance between the reflectors and the level of the applied voltage.
In addition to depending on the separation of the reflective surfaces, the peak transmission wavelengths also depend on the refractive index of the medium between the reflective surfaces. Thus, a tunable Fabry-Perot etalon can also be achieved by including between the reflective surfaces a material with a variable refractive index. An example of such a material is a liquid crystal, which can change refractive index in response to an electric field.